1. Field of the Invention
This invention relates to a total knee replacement prosthesis (TKR). Total knee replacement involves the surgical removal of the entire natural knee bearing surfaces and their replacement with artificial femoral and tibial components.
2. Description of Related Art
The invention is concerned with a type of TKR which includes femoral condylar surfaces which, to some extent, mimic the shape of the natural condyles and an interposed bearing component supported on a tibial platform. Condylar TKRs generally comprise (a) a femoral component having a pair of condylar surfaces, (b) a tibial component having a tibial platform fixed to the resected tibia, and (c) a bearing component usually of low friction plastic material interposed between the condylar surfaces and the tibial platform. The bearing component generally has dished surfaces for receiving the condylar surfaces of the femoral component. The bearing component can be made to be fixed onto the tibial platform or be rotatable and/or slidable in the anterior/posterior direction.
Stability of the artificial knee joint is provided by the dishing of the bearing surfaces and by the ligaments. In all cases, the collateral ligaments are required. In a fixed bearing design with dished bearing surfaces, the stability is sufficient particularly when there is a compressive force acting across the joint. In this situation, the cruciate ligaments are not necessary. For shallow bearing surfaces, which have the advantage of allowing extra freedom of motion, the posterior cruciate ligament is required. The above also applies to a mobile bearing design which only allows rotation. However, when anterior-posterior translation is allowed, the posterior cruciate ligament is required, no matter how dished are the bearing surfaces.
In the natural knee in extension, contact area is central on the tibial bearing surface or even anterior to the centre. As the knee is flexed, the contact area moves progressively posteriorly. This is important in that it provides an increasing lever arm to the quadriceps muscle in activities such as stair climbing and rising from a low chair, when there are high flexing moments acting on the knee. In addition, it is considered to be highly desirable for the femur to be displaced posteriorly with respect to the tibia at the higher flexion angles, since this will generally avoid posterior impingement of bone and soft tissues, allowing for a high range of flexion.
While proposals have been made in total knee design to provide for posterior displacement of the contact point with flexion, the corresponding problem of deliberately causing the contact point to be displaced anteriorly on extension, and the maintenance of stability during these movements has not been explored in prior proposals. The advantage of anterior displacement towards extension is an increase in lever arm of the hamstrings and gastrocnemius, thus helping to prevent hyperextension. It is, therefore, to a solution of these problems that the present inventions directed.